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Earthquakes Earthquakes factsheet pdf www.bgs.ac.uk/ask www.bgs.ac.uk Why do we have earthquakes? An earthquake occurs when rocks on either side of a weakness in the Earth’s crust (a fault) slip, causing the ground to vibrate and shake. Earthquakes commonly occur along plate boundaries where a lot of stress builds up in the rocks. Dir ec mo tion vem of ent Cr us 1 Am o of unt slip t The point where the slip begins is called the focus and the epicentre is the point on the Earth’s surface directly above the focus. Earth’s surface Epicentre Crust fic ci an a P ce Focus O 2 Upper mantle 1 School split by earthquake in 1964. Government Hill Elementary School, Prince William Sound, Alaska, USA US Geological Survey, Department of the Interior/USGS 2 Road split by earthquake in 1989. Loma Prieta, California, USA US Geological Survey, Department of the Interior/USGS 1 Unless otherwise stated, British Geological Survey ©NERC 2007. All rights reserved. www.bgs.ac.uk Earthquakes factsheet pdf www.bgs.ac.uk/ask When the blocks of rock slip suddenly a wave of intense vibrations travels outwards from the focus, we call these seismic waves. Epicentre Seismic waves Focus Fau lt Seismic waves can be recorded on a seismograph. The strength of an earthquake is referred to as its magnitude. There are several scales for measuring earthquake magnitude, one of which is the Richter scale. 1 Richter magnitude Earthquake effects Less than 3.5 Generally not felt, but recorded. 3.5 to 5.4 Often felt, but rarely causes damage. Under 6.0 At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings over small regions. 6.1 to 6.9 Can be destructive in areas up to about 100 km where people live. 7.0 to 7.9 Major earthquake. Can cause serious damage over larger areas. 8.0 or greater Great earthquake. Can cause serious damage in areas several hundred kilometres across. Famous earthquakes Armenia Spitak 1988 USA Prince William Sound 1964 Japan Kanto 1923 Magnitude 9.2 Magnitude 8.3 2 Magnitude 6.8 Portugal Lisbon 1755 Indian Ocean Sumatra-Andaman 2004 Magnitude 8.7 USA San Francisco 1906 Magnitude 8.3 Magnitude 9.1 1 Tokyo and Yokohama, Japan, 1923. US Geological Survey, Department of the Interior/USGS photo from the George A Lang collection 2 San Francisco-Oakland Bay Bridge, showing eartquake damage in 1989. US Geological Survey, Department of the Interior/USGS photo by C E Meyer For further information: www.bgs.ac.uk/education/earthquakes.html 2 Unless otherwise stated, British Geological Survey ©NERC 2007. All rights reserved.
